(a) Field of the Invention
The present invention relates to an organic electroluminescent (hereinafter, referred to as “EL”) display device, and a method for driving the organic EL display device. More specifically, the present invention relates to an organic EL display device capable of compensating for a reduction of the voltage between the gate and source of a driving transistor that occurs due to a voltage drop of the source voltage caused by the resistance component of a power source line, and a method for driving the organic EL display device.
(b) Description of the Related Art
In general, an organic EL display device is a display device that electrically excites a fluorescent organic compound to emit light, and drives N×M organic luminescent cells to display an image. Typically the techniques for driving the organic luminescent cells include, the passive matrix method and the active matrix method using thin film transistors (TFTs).
Compared with the passive matrix method that uses positive and negative electrodes lying at right angles to each other and selectively drives the electrode lines, the active matrix method connects TFTs and capacitors to the individual ITO (Indium Tin Oxide) pixel electrodes to maintain a voltage through capacitance.
FIG. 1 is a circuit diagram of a conventional pixel circuit for driving an organic EL device using TFTs, in which one of N×M pixels is shown.
Referring to FIG. 1, a P-type driving transistor M1 is connected to the organic EL device OELD to supply a current for emitting light. The current of the driving transistor M1 is controlled by a data voltage applied through a P-type switching transistor M2. Between the source and gate of the transistor M1, a capacitor Cst is connected for maintaining the applied voltage for a predetermined period of time. The gate of the transistor M2 is connected to the n-th scan line Scan[n], and the source of the transistor M2 is connected to the m-th data line Data[m].
Now, the operation of the above-constructed pixel circuit will be described. With a scanning signal applied to the gate of the switching transistor M2 to turn on the transistor M2, data voltage VDATA is applied to the gate (node A) of the driving transistor M1 via the data lines. As the data voltage VDATA is applied to the gate, the current flows to the organic EL device OELD via the transistor M1 to emit lights.
The current flowing to the organic EL device is given by the following equation:
                              I          OELD                =                                            β              2                        ⁢                                          (                                                      V                    GS                                    -                                      V                    TH                                                  )                            2                                =                                    β              2                        ⁢                                          (                                                      V                    DD                                    -                                      V                    DATA                                    -                                      V                    TH                                                  )                            2                                                          [                  Equation          ⁢                                          ⁢          1                ]            In the above equation, IOELD is the current flowing to the organic EL device; VGS is the voltage between the source and gate of the transistor M1 ; VDD is the source voltage applied to the source of the transistor M1 ; VTH is the threshold voltage of the transistor M1 ; VDATA is the data voltage; and β is a constant value.
As can be seen from Equation 1, the current corresponding to the data voltage VDATA applied to the pixel circuit shown in FIG. 1 is sent to the organic EL device OELD, which then emits light. Here, the data voltage VDATA has a multilevel value in a predetermined range, for representing gradation.
According to the conventional pixel circuit, virtually all the source voltage VDD is applied to the source of a driving transistor M1 that is closely connected, via a power source line, to an external source outputting the source voltage VDD. But a voltage VDD′ that is lower than the source voltage due to the resistance component of the power source line is applied to a source of the driving transistor M1 that is connected far away from the external voltage source via the power source line.
This can be described as follows in further detail with reference to FIGS. 2 and 3.
In the pixel circuit of FIG. 2, it is assumed that an external power source (not shown) is positioned adjacent to the first row of the pixel circuit.
In FIG. 2, the source voltage VDD is applied directly to the driving transistor M1 of the pixel circuit in the first row, and, via a resistance Rp, to the driving transistor of the pixel circuit in the n-th row.
Assuming that data voltage V1 is applied to the gate of the driving transistor of the pixel circuit in the first row and data voltage V2 is applied to the gate of the driving transistor of the pixel circuit in the n-th row, the driving transistor M1 is turned on as shown in the equivalent circuit diagram of FIG. 3.
As shown in FIG. 3, the voltage VDD is applied to the source (denoted by ‘A’) of the driving transistor of the pixel circuit in the first row, but the voltage VDD′ that is lower than VDD is applied to the source (denoted by ‘B’) of the driving transistor of the pixel circuit in the n-th row due to a voltage drop caused by the resistance Rp.
Accordingly, when the same data voltage is applied in order to represent the same gradation in the first and n-th rows, i.e., V1=V2, the voltage VDD applied to the source of the driving transistor in the first row differs from the voltage VDD′applied to the source of the driving transistor in the n-th row. Hence a current of a different magnitude flows to the organic EL device as can be seen from Equation 1. Thus the conventional organic EL display device exhibits different gradations according to the position of the pixel even with the same data voltage, and therefore it has difficulty in accurately representing gradation.
Particularly, the difference of the source voltage caused by the resistance component of the power source line becomes greater with an increase in the distance from the external voltage source, and, for a high resolution (greater than SVGA) organic EL display device, a current of up to several amperes flows to the whole panel during a full white driving operation, resulting in a deterioration of the luminance by scores of grays.